1. Field of the Invention
This invention relates generally to railway signaling systems, and it relates more particularly to means useful on electrified railroads for allowing trains safely and automatically to traverse gaps in the overhead electric power distributing conductor.
An electrically driven land vehicle such as an electric locomotive or a self-propelled transit car is equipped with a current collector that maintains sliding or rolling contact with a wayside conductor extending parallel to the rails or guideway defining the vehicle's path of movement along the railroad right of way. Typically, the wayside conductor is either in overhead catenary, in which case the current collector is a pneumatic or spring-loaded pantograph on the roof of the vehicle, or a third rail, in which case the current collector is a "shoe" in sliding contact with the third rail. In either case, the collector provides on board the vehicle a voltage whose magnitude is determined by the voltage magnitude of a source of electric power that energizes the particular section of the wayside conductor with which the collector is cooperating. This voltage is applied to the input of controllable power conditioning means which in turn supplies electric power to electric traction motors for propelling the vehicle along the right of way at the desired speed and in the proper direction. Where the wayside power is characterized by alternating current (a-c) rather than direct current (d-c), the power conditioning means on board the vehicle usually includes a single-phase voltage stepdown main transformer.
The wayside conductor is part of an electric power distribution system that is fed from a plurality of sources. Such sources may comprise the railroad's own low voltage d-c or a-c power generating plants located near the right of way, or they may comprise electric utility-owned substations that supply highvoltage 3-phase a-c power at commercial power frequencies (e.g., 60 Hz in the United States, 50 Hz in Europe). Where 3-phase power is used, it is common practice to feed adjacent sections of the wayside conductor from different phases of the source, and at predetermined buffer zones or transition points along the right of way the adjacaent sections are electrically insulated from one another to avoid short circuiting the two different phases. At each such "phase break," a neutral or "dead" (i.e., non-conducting) segment is provided in the wayside conductor to bridge a gap between the adjacent "live" (i.e., energized) sections thereof.
Such dead segments or gaps in the wayside conductor are also required on electrified railroad systems where adjacent sections of the conductor are respectively energized by different power sources characterized by unequal frequencies and/or diverse voltage magnitudes. For example, d-c and a-c sources might be mixed on a given railroad line, or adjacent sections of a totally d-c line might have different voltage magnitudes. In another practical example, there is a plan to improve the "Northeast Corridor" of the United States, which refers to the electrified railroad extending from Washington, D.C. to New York City and beyond, by converting its mainline catenary system from the existing single-phase, 25 Hz, 11 KV, a-c power to commercially available 60 Hz power at either 12.5 KV or 25 KV. However, most of the electrified branch lines of this Corridor will not be so converted. Consequently, the two new sources of power (60 Hz, 12.5 KV and 60 Hz, 25 KV) and the old source (25 Hz, 11 KV) will be comingled in the Northeast Corridor, and in the future the electric locomotives or self-propelled transit cars operating in this environment will experience transitions between some or all possible combinations of these different sources.
An electrically propelled traction vehicle operating on an electrified railroad with diverse power sources should have means for anticipating each transition point so that, as it reaches a gap in the wayside conductor, the vehicle can coast through the gap in an unpowered state. This will avoid damage that might otherwise occur if an electric arc were drawn as the current collector on the vehicle separates from a live section of the wayside conductor on entering the dead segment thereof at the transition point. If adjacent sections of the wayside conductor are energized by sources having different voltage magnitudes, the vehicle's power conditioning means should also be appropriately adjusted or reconnected before the propulsion system is repowered so as to avoid damage in the event the transition has been from a low voltage section to a high voltage section of the conductor. Several known systems for accomplishing these results will now be briefly summarized.
2. Description of the Prior Art
In accordance with one prior art strategy for traversing a voltage changeover gap between adjacent sections of a wayside conductor, the desired sequence of operations is performed manually by the operator of the vehicle. When alerted by appropriate signals indicating that the vehicle is about to reach a transition point between two different wayside voltages (both of which are known), the operator first shuts down the power conditioning means, then opens a circuit breaker connected between the current collector and the power conditioning means, then actuates either a tap changer associated with the secondary winding of the main transformer (in a-c systems) or a series/parallel changeover switch associated with each pair of motors (in d-c systems), and finally, after the vehicle has coasted beyond the transition point, recloses the circuit breaker. Such manual operations are obviously impractical for multiple unit trains and for any railroad line having successive phase breaks spaced relatively close together.
In U.S. Pat. No. 3,957,236 granted on May 18, 1976, to D. R. Phelps and P. T. Ryan and assigned to the General Electric Company, there is described and claimed an improved method of traversing a predetermined transition point between successive diverse wayside power sources. This method comprises the steps of automatically opening a circuit on board an electrically propelled vehicle as the vehicle approaches the transition point to disconnect the vehicle from the wayside power source on the side of the transition point from which it is approaching, changing a power switch on board the vehicle from a first power source position to a second power source position (if required), and automatically closing the circuit when the vehicle passes to the opposite side of the transition point, thereby connecting the vehicle to the next wayside power source. To indicate when the vehicle is in the vicinity of the transition point, a pair of signal devices are placed along the right-of-way on opposite sides of that point. These devices, as disclosed by Phelps and Ryan, are active circuit elements, and one can transmit a different signal than the other so that a cooperating receiver on board the vehicle can differentiate the one from the other as the vehicle passes their respective locations. Thus, the receiver not only detects the position of the vehicle with respect to the transition point but also indicates the voltage magnitude of the wayside power source at that position. Signal frequencies need to be carefully selected so as to avoid conflicts with existing automatic cab signaling systems.
Another automatic control system, heretofore known in England, uses passive trackside magnets instead of active signal transmitting devices. As the vehicle approaches a transition point, a magnetic field detector on board the vehicle passes over a first one of the magnets and activates relays that open a traction power circuit breaker and that reset the detector. On leaving the transition point, the same detector passes over the other magnet (which has the same polarity as the previous one), and this time it responds by activating relays that initiate reclosure of the circuit breaker and again reset the detector. Means is provided on the vehicle for sensing the actual magnitude of voltage on the new section of wayside conductor when the vehicle leaves the transition point, and before the circuit breaker is allowed to release this sensing means actuates a voltage changeover switch (if required) to reconnect the main transformer in accordance with the sensed voltage magnitude.